1. Field of the Invention
The present invention relates generally to the smelting of aluminum metal from alumina, and more specifically to using acid to convert aluminum hydroxide starting materials into positively charged alumina for increased solubility in a Hall-Heroult electrolyte.
2. Description of the Prior Art
Practically all the aluminum metal smelted from raw materials is made by the Hall-Heroult process invented in the nineteenth century. See, Ernest W. Dewing, "The Thermochemistry of Aluminum Smelting", pp. 341-350, Proc. of the Savard/Lee Intl. Symp. on Bath Smelting, The Materials and Metals Society (Canada), 1992. Such process uses very high electrical currents to electrolyze alumina, Al.sub.2 O.sub.3, which is dissolved in an electrolyte of molten cryolite, Na.sub.3 AlF.sub.6, at temperatures of 945.degree. C. to 975.degree. C. But such high temperatures can cause the carbon in the anodes to burn with the air rather than contribute to the removal of oxygen from the alumina.
A consumable carbon anode is used in the Hall-Heroult process with a gross cell cathode-anode voltage of about 4-5 volts. The typical ohmic resistance of the electrolyte is about 0.4 ohms/cm, and a typical current density of 0.75 amps/cm.sup.2 produces a voltage drop of about 0.3 volts/cm.sup.-1. The ohmic drop is thus about 1.2-1.5 volts, the reversible EMF is about 1.2 volts, the kinetic overpotential is about 0.5 volts, and the gas-bubble layer resistance under the anode drops about 0.15 volts. This gives a total of about 3.2 volts that is dropped in the inter-electrode gap. About another volt is lost within the anode and cathode electrodes and their busbar connections. Typically, over thirteen kilowatts of electrical power per kilogram of aluminum metal is needed, and this will result in about 0.45 kilograms of carbon being consumed from the carbon anode.
A typical modern smelting cell will draw about 200,000 amperes and the electrical energy consumed in the cells contributes to both the Gibbs energy needed by the chemistry and the ohmic heating that keeps the electrolyte hot. The overall reaction approximates to 2Al.sub.2 O.sub.3 +3C+energy=4Al+3CO.sub.2.
Alumina has been used as the primary feed material in the electrolytic smelting of aluminum metal for over a hundred years. Bauxite, in particular, is the raw material that is universally used. Worldwide, over forty million tons per year of smelting alumina is produced and this, in turn, yields twenty million tons of aluminum metal. The Bayer process is the principle method now used to convert bauxite to alumina, and such process depends on a caustic (e.g., NaOH) to leach the bauxite. Such use of a caustic yields negatively charged alumina. The present inventor, John S. Rendall, has determined that such negative charges impair the dissolution of alumina (and the rate thereof) in the cell electrolyte, and require more electrical power to drive the electrolysis than would be required if such charges were positive or neutral.
Alumina that is good enough to be used for the electrolytic smelting of aluminum is typically referred to as "cell-grade alumina". One of the principle characteristics important to cell-grade alumina is its relative solubility in molten fluoride salt electrolyte. Universally, such molten fluoride salt electrolytes are heated to 950.degree. C. just to raise the solubility to a approximately four percent by weight.
The optimal alumina reactivity and the optimum electric voltage needed to produce a useful electrolytic dissociation of the alumina has been the subject of a great deal of scientific study. Just about all electrolytic cells use engineered alumina precipitated as a hydroxide from caustic solutions of approximately nine pH. The low degree of alumina dissolution and the rate thereof in the molten bath electrolyte is an on-going problem. About four percent alumina, Al.sub.2 O.sub.3, by weight, is considered the upper limit at 950.degree. C. The usual way that alumina is fed into cells produces a lot of dust. Such alumina feed is also used in fluid beds to capture fluoride emissions. The voltage drop of four to six volts across a conventional electrolytic cell includes the bath resistance, the electrode resistance of two electrodes, as well as the energy of electrolysis ameliorated by the electrolytic formation of CO.sub.2.
The dissolution of the alumina in the molten bath is so low, it requires careful and sophisticated replenishment. At 750.degree. C. the dissolution and rate thereof of the alumina is less than one percent and cannot be used at this temperature. Only six percent, by weight, is usually possibly at 950.degree. C. Over-feeding of alumina will create a bottom sludge that can cover and electrically isolate the molten aluminum cathode surface. This will cause a reduction in the electrical current that can be induced due to the increased voltage required, and thereby cause the cell to freeze up because it cannot produce enough electrical heating. At under one percent, by weight, alumina in the bath causes an increase in the voltage drop that occurs in the carbon anode and reduces the power input (amps) causing the cell to freeze. This localizes heat generation there and adversely affects the crust seal at the top of the cell. This localized heating at the carbon anode can also be responsible for the production of carbon fluoride gases.
In the preparation of alumina for use in cells, any alumina that precipitates as aluminum hydroxide from a sodium aluminate solution is usually considered to comprise negatively charged ions, Al(OH).sub.4.sup.-. Such precipitation is usually done at a pH of about nine, and a temperature of about 80.degree. C. The negatively charged ions are produced by caustic leaching of bauxite, and contribute to a clustering of crystals into larger particles. The bonding mechanisms in these clusters consumes most of the negative ionic charges, and the overall negative charge is almost completely neutralized. See, Karl Wefers Chanakya, "Oxides and Hydroxides of Aluminum", Alcoa Laboratories, 1987. In any event, an intermediate aluminum hydroxide must be aged 24-hours to get the type of alumina that will work well in smelters. The day-old aluminum hydroxide is then dried and calcined at about 1000.degree. C. to produce the desired cell-grade alumina.